Sensor unit device and method for avoiding condensation on a surface

ABSTRACT

The invention relates to a sensor unit for an apparatus for preventing condensation of a gas, particularly water vapour, on a surface of an object, with a temperature measuring device for measuring an object temperature, with a dew point determination device for determining a dew point temperature of the gas in an atmosphere surrounding the object and with a regulating and control device operatively connected to the temperature measuring device and the dew point determination device and with which an adjusting device for increasing a temperature difference between the object temperature and the dew point temperature is controllable as a function of the data obtained by the temperature measuring device and the dew point determination device in such a way that a lowering of the object temperature to or below the dew point temperature is prevented. The sensor unit is inventively characterized in that the dew point determination device is constructed as a dew point sensor for the direct measurement of the dew point and the temperature measuring device is constructed as a temperature sensor operating in contactless manner. The invention also relates to an apparatus and a method for preventing condensation of a gas, particularly water vapour, on a surface of an object.

The present invention relates to a sensor unit according to the preambleof claim 1 and to a method for preventing condensation of a gas,particularly water vapour, on a surface of an object according to thepreamble of claim 12. The invention also relates to an apparatus forpreventing condensation of a gas, particularly water vapour, on asurface of an object.

A sensor unit according to the preamble for an apparatus for preventingcondensation of a gas, particularly water vapour, on a surface of anobject has the following components: a temperature measuring device formeasuring an object temperature, a dew point determination device fordetermining a dew point temperature of the gas in an atmospheresurrounding the object and a regulating and control device which isoperatively connected to the temperature measuring device and the dewpoint determination device and with which it is possible to control anadjusting device for increasing a temperature difference between theobject temperature and the dew point temperature as a function of dataobtained by the temperature measuring device and the dew pointdetermination device in such a way that a reduction of the objecttemperature to or below the dew point temperature is avoided.

In a method according to the preamble for preventing condensation of agas, particularly water vapour, on a surface of an object, the followingmethod steps are performed: (a) measuring an object temperature, (b)determining a dew point temperature of the gas in an atmospheresurrounding the object and (c) raising the object temperature and/orreducing the dew point temperature as a function of the objecttemperature measured in step (a) and/or the dew point temperaturedetermined in step (b) for avoiding a lowering of the object temperatureto or below the dew point temperature.

Such sensor units or such a method can be used in process engineering,as well as in automotive engineering. In the latter it is of greaterimportance to prevent condensation, particularly of water vapour, onsurfaces. For example in vehicle traffic dangerous situations can arisedue to the “misting” of the windscreen.

Hitherto such sensor units have been implemented with capacitivehumidity or moisture sensors or those based on a conductivitymeasurement, in conjunction with a contacting temperature sensor.Capacitive humidity sensors determine as the actual measured quantitythe “relative humidity”, i.e. a measured quantity which can bedetermined when knowing the temperature of the partial pressure of watervapour and therefore the dew point temperature of the gas. The basis forthis determination is the link between the vapour pressure p_(D) and thedrying temperature at a specific relative humidity. All these curves areconventionally plotted in a “hx” graph. In the case of the same vapourpressure p_(D), i.e. with the same water vapour content x (in g/kg), themeasured gas has different “relative humidities” at different dryingtemperatures.

The dew point temperature, which can be determined from the 100%relative humidity curve is decisive for the “misting” or condensation ona surface.

As the “relative humidity” curves, also referred to as RH curves and thedrying temperature as a function of the vapour pressure at a specificrelative humidity have a very considerable gradient in the range below0° C., there is a marked reduction in the sensitivity of the dew pointdetermination in this range.

Capacitive humidity sensors also suffer from the disadvantage of drift,i.e. they are not stable on a long term basis. Such drift moreparticularly occurs in the case of high and very low humidity levels,which is attributed to saturation or drying effects.

Finally, capacitive humidity sensors are susceptible to dirtying, whichis e.g. particularly disadvantageously noticeable if smoking occurs in apassenger compartment of a car.

The object of the invention is to provide a sensor unit and a method ofthe aforementioned type usable in a variable manner and in whichcondensation can be reliably prevented. The sensor unit must also beparticularly inexpensive to manufacture.

This object is achieved by a sensor unit having the features of claim 1and a method having the features of claim 12.

Advantageous developments of the sensor unit according to the inventionand preferred developments of the method according to the invention formthe subject matter of subclaims.

A sensor unit of the aforementioned type is inventively furtherdeveloped in that the dew point determination device is constructed as adew point sensor for the direct measurement of the dew point and thatthe temperature measuring device is constructed as a temperature sensoroperating in contactless manner.

Correspondingly a method of the aforementioned type is inventivelyfurther developed in that the dew point temperature of the gas ismeasured directly with a dew point sensor and that the objecttemperature is measured in contactless manner.

A first basic concept of the present invention is that the dew pointtemperature is no longer indirectly determined as hitherto by measuringthe relative humidity, but instead with the aid of a dew point sensorthe dew point temperature is directly measured. As uncertainties whendetermining the relative humidity no longer play a part for the dewpoint temperature, condensation on the object surface can be much morereliably prevented.

A second basic concept of the invention relates to the measurement incontactless manner of the object temperature. The inventive sensor unitand inventive method are consequently usable in a particularly variablemanner. For example, a moving object can also be monitored andcondensation on its surface avoided.

The contactless temperature measurement has the advantage that no energyis removed from the measured object. This is particularly advantageousif the temperature of a surface is to be determined, because in the caseof surface temperature measurements frequently the problem arises thatthe sensor element used removes energy from the surface and this leadsto erroneous measurements. In addition, as a result of the contactlesstemperature measurement it is possible to select a field of measurementthrough the suitable choice of a difference and an angular aperture, sothat e.g. an integral measurement of the surface is also possible. Amonitoring of moving objects can in particular be important forindustrial manufacturing processes.

The sensor unit according to the invention can be inexpensivelymanufactured and can be produced by mass production in large numbers ata low price.

A wetting sensor is preferably used as the dew point sensor. This can bea measuring component where the wetting of a measurement surface withthe gas in question, i.e. the condensation of said gas, is measured.This leads to the advantage that the state on the object surface wherecondensation is to be prevented is simulated in the dew point sensor.This particularly reliably prevents condensation on the object surface.

In particularly preferred manner the dew point sensor is constituted bya sensor in which the measuring principle makes use of the change to alight reflection and/or light scattering, particularly an internalreflection, when gas is condensed on a measurement surface.

Such sensors are e.g. known from DE 199 32 438 and in the case of acompact, inexpensive structure have a very low dirtying sensitivity andat the same time an easy cleaning possibility. The opticalcharacteristics of a surface, particularly its reflectivity change verygreatly if said surface is wetted with a gas, i.e. if said gas condenseson the surface. This permits a very precise determination of the dewpoint temperature.

Particular preference is given to a sensor in which the change to aninternal reflection due to condensation of the measurement gas on themeasurement surface is measured, because this reflection change islargely independent of possible dirtying, such as e.g. by dust, on themeasurement surface.

The temperature sensor can be constituted by an infrared sensor and inprinciple use can be made of any detector suitable for the infraredspectral range, e.g. a photoconductive cell, a thermocouple, a bolometeror a semiconductor detector, such as e.g. a photodiode. However,preferably a thermopile detector is used. Such detectors are obtainableat a low cost and allow a precise temperature measurement.

The precision of the temperature measurement can be further increased ifthe temperature sensor is provided with a spectral filter. This can inparticular be an 8 to 14 μm window, i.e. an atmospheric window.

There can also be a further temperature measuring device for determiningthe temperature of the atmosphere surrounding the object. This inparticular relates to the determination of the temperature in a motorvehicle passenger compartment. With a corresponding regulating deviceand using the measured interior temperature, assuming a correspondingdew point gap, the climatic conditions in the passenger compartment canbe regulated to the comfort range, which leads to considerableadvantages for the occupants.

In a particularly preferred development the inventive sensor unit ishoused in a common housing. Such a compact structure permits multipleuses and easy replacement of the sensor unit.

The invention also relates to an apparatus for preventing thecondensation of a gas, particularly water vapour, on a surface of anobject, which has a sensor unit according to the invention, as well asan adjusting device for increasing a temperature difference between theobject temperature and the dew point temperature.

With such an apparatus or system the advantages explained in conjunctionwith the inventive sensor unit are achieved.

The adjusting device can be constructed as a heating device. It can be adevice for the direct heating of the object, such as e.g. a rear windowheater and/or a device for the indirect heating of the object, such ase.g. a heater blower.

If for particular, e.g. process engineering reasons, a heating of theobject is undesired, an increased temperature difference between theobject temperature and dew point temperature can be brought about bylowering the dew point temperature. In this case the adjusting device ispreferably constructed as a drying device for reducing a gas content,particularly a water vapour content, in the atmosphere surrounding theobject.

The apparatus according to the invention can in particular be used forpreventing the misting of the windows of a motor vehicle. As a result ofthe aforementioned, fundamental differences between the sensor unitaccording to the invention and the prior art and the advantagesobtained, with such an apparatus it is possible to particularly reliableprevent condensation of water vapour on windows, i.e. “misting”, andconsequently the safety of the occupants is significantly increased.

From the regulation and control standpoint, the control of the adjustingdevice by the regulating and control device preferably takes place insuch a way that the temperature difference between the objecttemperature and the dew point temperature is kept above a predeterminedminimum temperature difference.

Further advantages and characteristics of the sensor unit and methodaccording to the invention are described hereinafter relative to theattached diagrammatic drawings, wherein show:

FIG. 1 A diagrammatic view of an inventive apparatus with an inventivesensor unit.

FIG. 2 A diagrammatic view of a dew point sensor such as can be used inthe sensor unit according to the invention.

The apparatus shown in FIG. 1 has a sensor unit 10 according to theinvention and an adjusting device 18, which can e.g. be a heater bloweror a rear window heater. With the aid of a temperature sensor 40 as thetemperature measuring device 12, which can be a thermopile sensor, thesurface temperature of an object 20 is determined. The temperaturesensor 40 establishes in contactless manner the infrared radiation of ameasurement spot 22 on the surface 21 of the object 20 and an acceptancerange of the temperature sensor 40 is diagrammatically represented by anacceptance cone 13. The temperature sensor 40 is connected to aregulating and control device 16.

The sensor unit 10 also has a dew point sensor 50 as a dew pointdetermining device 14 and this is also connected to the regulating andcontrol device 16. The dew point sensor 50 is used for determining thedew point temperature of a diagrammatically represented gas 28, whichcan in particular be water vapour and which surrounds the object. Thedew point sensor 50 is preferably constructed as a wetting sensor and inparticular as a sensor of the type described in DE 199 32 438.

The temperature sensor 40, dew point sensor 50 and regulating andcontrol device 16 are housed in a common housing 26, which ensures avery compact structure. The adjusting device 18, which can e.g. be aheater blower, but also a rear window heater, is controlled by theregulating and control device 16 in such a way that condensation of thegas 28, e.g. water vapour, on the surface 21 of the object 20 isprevented. The measured surface temperature serves as a guide quantity.

If there is a critical temperature difference between the objecttemperature and the dew point temperature which leads to a condensationrisk, corresponding corrective measures must be performed.

The following can be carried out as corrective measures:

a) heating the object 20 (temperature difference between objecttemperature and dew point temperature increased);

b) “drying” the atmosphere surrounding the object (dew point temperaturedrops, i.e. the temperature difference between the object temperatureand dew point temperature increases);

c) indirect heating of the object by heating the gas (effect as in a));or

d) a combination of a) to c).

As a result of the determination of the actual (real) dew pointtemperature it is possible to perform several actions in a targetedmanner. In the case of additional knowledge, e.g. of the interiortemperature of a passenger compartment, assuming a correspondingtemperature difference with respect to the dew point, the climaticconditions can be regulated to the comfort range, which leads toconsiderable advantages for the occupants.

If a simple anti-misting device is required, it can be advantageous tointroduce a ADT control and for this purpose the object temperature isused as the guide quantity. The dew point sensor 50 is regulated to atemperature which is below the object temperature by the ΔDT value (e.g.5° C.). As soon as misting of the dew point sensor 50 occurs, actions a)to d) are performed. The actions can differ as a function of the objecttemperature.

FIG. 2 shows a dew point sensor of the type used in preferred manner inthe sensor unit according to the invention.

The essential component of this sensor is an arrangement of a lightguide 52 into which light 56 is coupled from a transmitter or a source54, which can e.g. be a light emitting diode. Following a plurality ofinternal reflections on the outer faces of the light guide 52 coupledout light 66 reaches a receiver 68, which can be a photodiode. A Peltierelement 74 is applied to the back of the light guide 52 enabling thelatter to be cooled in a defined manner.

During the measurement the Peltier element 74 cools the light guide 52until a gas 28 to be tested, which can in particular be water vapour,condenses on an outer surface 60 of the light guide 52. Such acondensation coating 58 is diagrammatically illustrated in the left-handarea of surface 60 of light guide 52. Through the wetting of the surface60, e. with water, there is a rise in the critical angle for theinternal reflection above the incidence angle of the light 56 withrespect to the surface normal of the surface 60, so that unlike thesituation as hitherto, the light is no longer totally reflected on theinner interface and is instead coupled out of the light guide 52. As aresult of this fraction of coupled out light 62, the intensity detectedin the receiver 68 drops and consequently it can be concluded that thereis wetting of the surface 60 and that the dew point temperature has beenreached.

The sensor 50 shown in FIG. 2 has as a particular advantage that dirtparticles 64 virtually lead to no deterioration of the measurementprecision, because if said dirt particles are dry, due to theirnegligible contact face with the surface 60 of light guide 62 comparedwith the total area, they only bring about a change to the criticalangle for the total reflection in a negligible area percentage.

The overall dew point sensor 50 is compactly housed in a transistorhousing 70, on whose underside are provided terminals 72 for controllingthe transmitter 54, Peltier element 74 and for reading out a signal ofreceiver 68.

The dew point sensor 50 shown is characterized by a very small, compactconstruction, which is designed for mass production, as well as byrecyclability. Due to the measuring principle used of a reflectionchange during condensation on a measurement surface, it constitutes aprimary method, where there is no calculating back to the quantity to bedetermined, here the dew point temperature, so that a high precision canbe achieved. Ageing phenomena are minimal with such a sensor, e.g. whencompared with capacitive sensors. The sensor also actively simulateswhat would take place on the window at a corresponding temperature, i.e.possibly misting.

As a result of the measuring principle used the dew point sensor 50 hasa very good long term stability, so that recalibrations are unnecessary.A low-maintenance and more maintenance-friendly operation is obtained asa result of the aforementioned significant insensitivity to dirtying andalso by the ease of cleaning the sensor. These advantageouscharacteristics of the dew point sensor 50 consequently permitmeasurements, particularly also in situ measurements in dust, granules,such as e.g. cereals, etc.

The sensor can be used between −40 and +100° C. When using light guidesin place of the transceiver, the temperature range can be furtherincreased and in this case the Peltier element is a limiting factor.

This also defines the humidity or moisture use range. As the sensorprinciple is based on saturation, it is always adapted to 100% RH.

Possible uses of the sensor unit and method according to the inventionare, in addition to process, air conditioning, medical and foodengineering, in particular automotive engineering, as has been describedhereinbefore. There are also uses in the aeronautical and astronauticalindustries, as well as in the quality control field.

1. Sensor unit for an apparatus for preventing the condensation of agas, particularly water vapour, on a surface of an object, with atemperature measuring device (12) for measuring an object temperature,with a dew point determination device (14) for determining a dew pointtemperature of the gas in an atmosphere surrounding the object (20) andwith a regulating and control device (16) operatively connected to thetemperature measuring device (12) and the dew point determination device(14) and with which an adjusting device (18) for increasing atemperature difference between the object temperature and the dew pointtemperature can be controlled as a function of the data obtained by thetemperature measuring device (12) and the dew point determination device(14) in such a way that a reduction of the object temperature to orbelow the dew point temperature is prevented, characterized in that thedew point determination device (14) is constructed as a dew point sensor(50) for the direct measurement of the dew point and the temperaturemeasuring device (12) is constructed as a temperature sensor operatingin contactless manner.
 2. Sensor unit according to claim 1,characterized in that the temperature sensor is constructed as aninfrared sensor.
 3. Sensor unit according to claim 2, characterized inthat the temperature sensor is a thermopile sensor.
 4. Sensor unitaccording to claim 2, characterized in that the temperature sensor isprovided with a spectral filter.
 5. Sensor unit according to claim 1,characterized in that the dew point sensor (50) is of the type in whichthe measuring principle is the change to a light reflection and/or lightscattering, particularly an internal reflection, when the gas iscondensed on a measurement surface (52).
 6. Sensor unit according toclaim 1, characterized in that there is a further temperature measuringdevice for determining the temperature of the atmosphere (28)surrounding the object (20), particularly the temperature within a motorvehicle passenger compartment.
 7. Sensor unit according to claim 1housed in a common housing (26).
 8. Apparatus for preventing thecondensation of a gas, particularly water vapour, on a surface of anobject, having a sensor unit (10) according to claim 1, and with anadjusting device (18) for increasing a temperature difference betweenthe object temperature and dew point temperature.
 9. Apparatus accordingto claim 8, characterized in that the adjusting device is constructed asa heating device for the direct and/or indirect heating of the object.10. Apparatus according to claim 8, characterized in that the adjustingdevice is constructed as a drying device for reducing a gas content,particularly a water vapour content, in the atmosphere surrounding theobject.
 11. Apparatus according to claim 8, characterized in that it isconstructed as a means for preventing the misting of the windows of amotor vehicle.
 12. Method for avoiding the condensation of a gas,particularly water vapour, on a surface of an object, with the methodsteps of: (a) measuring an object temperature, (b) determining a dewpoint temperature of the gas in an atmosphere surrounding the object,(c) raising the object temperature and/or reducing the dew pointtemperature as a function of the object temperature measured in step (a)and/or the dew point temperature determined in step (b) for preventing alowering of the object temperature to or below the dew pointtemperature, characterized in that the dew point temperature of the gasis directly measured with a dew point sensor and the object temperatureis measured in contactless manner.
 13. Method according to claim 12,characterized in that the temperature difference between the objecttemperature and dew point temperature is kept above a predeterminedminimum temperature difference by a regulating and control device (16).